Liquid crystal displays are thin and low power-consuming and for these reasons are used extensively for example, in office automation equipment, such as word processor devices and personal computers; personal digital assistants, such as electronic note pads; and VTRs with a built-in camera and a liquid crystal monitor.
Liquid crystal panels do not emit light as CRTs (Cathode Ray Tubes) and EL (electroluminescense) displays. Instead, some liquid crystal panels include in the back thereof a section containing a so-called backlight, which is a fluorescent tube, and produces a display by controlling the transmission/interruption of the light emitted by the backlight. These liquid crystal displays are termed transmissive types.
Under very bright ambient light, the transmissive liquid crystal display cannot provide satisfactory visibility because of its relatively dark appearance. In addition, the backlight consumes so much power that it typically accounts for more than half the total power consumption of the liquid crystal display.
To solve these problems, the personal digital assistant, which is often carried and used outdoors, incorporates a reflective liquid crystal display with a reflector plate in place of a backlight to produce a display by controlling the transmission/interruption of the light reflected off the reflector plate. Some reflective liquid crystal displays include a polarizer plate and operate in TN (twisted nematic) mode or STN (super twisted nematic) mode, while others operate in phase change guest-host mode without a polarizer plate and therefore can produce a bright display. Extensive research and development is being performed recently on the phase change guest-host mode.
Conversely to the transmissive liquid crystal display, the reflective liquid crystal display, which depends on reflected light for a display, cannot provide satisfactory visibility under poor ambient illumination. Further, if the reflector plate is provided outside the glass substrates that sandwich a liquid crystal layer in the liquid crystal display, an overlapping, slightly displaced image occurs because of the parallax between the liquid crystal layer and the reflector plate and degrades the display quality of the true image. In addition, if an undulating reflector plate is provided between the glass substrates so as to be in contact with the liquid crystal layer, the undulations of the reflector plate make it difficult to maintain a preferably uniform distance between the glass substrates.
To address the poor visibility of the transmissive liquid crystal display and the reflective liquid crystal display, a liquid crystal display incorporating a transmissive reflector film and a hybrid liquid crystal display that operates both as a transmission type and a reflection type are being developed. The latter, hybrid type includes a backlight and a reflector plate part of which has holes so that the reflector plate can transmit the light emitted by the backlight. The hybrid type switches between reflection and transmission mode by turning on/off the backlight, hence operating as a transmissive type when ambient light is insufficient and as a reflective type when ambient light is sufficient. The reflector plate has undulations that scatter light when operating in reflection mode to produce a bright display.
The liquid crystal cell in the liquid crystal display is constructed of two substrates disposed opposite each other to provide a gap to inject liquid crystal. The substrates are made of glass or other material. The distance by which the opposing substrates are separated, that is, the cell gap, is typically maintained by spherical particles, or so-called spacers, that are provided randomly all over the substrate.
For example, Japanese Laid-Open Patent Application No. 11-101992/1999 (Tokukaihei 11-101992; published on Apr. 13, 1999) discloses a transmission/reflection hybrid liquid crystal display having TFTs 111, pixel electrodes 102, and an interlayer insulation layer 110 on a substrate 105 as shown in FIG. 9. On the opposite substrate 104 are deposited a color filter 107 with a BM 106, a transparent electrode 108, and alignment layer 109 in this order.
The substrate 105 and the opposite substrate 104 are located opposite each other and separated by spherical spacers 103. Liquid crystal is injected between the two substrates to form a liquid crystal layer 101. In each pixel, there is provided a pixel electrode 102 divided into a reflection region 102a made of a metal film and a transmission region 102b made of ITO. The cell gap, that is, the gap where liquid crystal is injected, is half as large over the reflection region 102a of the pixel electrode 102 as over the transmission region 102b. A desired cell gap is provided, for example, by means of the spherical spacers 103 that are distributed in the layer in suitable numbers.
The structure in which the cell gap over the reflection region 102a is reduced to half that over the transmission region 102b prevents an overlapping, slightly displaced image from occurring and thus degrading the display quality of the true image. Another advantage of the structure is that the undulations of the reflection region 102a do not negatively affect the cell gap because the reflection region 102a is located on the interlayer insulation layer 110.
According to the foregoing patent application, the cell gap is practically dictated by the diameter of the spherical spacers 103. Therefore, if the density of the spherical spacers 103 varies from place to place on the substrates 104 and 105, the cell gap also varies. Stably controlling the density of the spherical spacers 103 is difficult and results in the following problems.
As shown in FIGS. 10(a), 10(b), the cell gap varies greatly depending on the size and density of the distributed spherical spacers 103. FIG. 10(a) is an example of relatively many spherical spacers 103 being distributed, whereas FIG. 10(b) is an example of relatively few spherical spacers 103 being distributed for comparison.
In these examples, d1>d2 where d1 is the cell gap over the reflection region when there are relatively many spherical spacers 103 being distributed (see FIG. 10(a)), and d2 is the cell gap over the reflection region when there are relatively few spherical spacers 103 being distributed (see FIG. 10(b)).
Further, d3>d4 where d3 is the cell gap over the transmission region when there are relatively many spherical spacers 103 being distributed (see FIG. 10(a)), and d4 is the cell gap over the transmission region when there are relatively few spherical spacers 103 being distributed (see FIG. 10(b)).
As can be understood from these examples, a stable cell gap is difficult to obtain with the use of the spherical spacers 103 because the cell gap is affected by the density of the spherical spacers 103. Besides, the density is difficult to control stably.
Even if the spherical spacers 103 are uniformly distributed, the spherical spacers 103 whose size is adjusted for the smaller cell gap are too small to provide a suitable cell gap in regions where the cell gap must be larger.
If the spherical spacers 103 are distributed in great numbers in consideration of these problems, the spherical spacers 103 aggregate. The aggregation of the spherical spacers 103, especially, in regions where the cell gap is small, causes those spherical spacers 103 to be pressed and carve into the electrodes sandwiching the spherical spacers 103, i.e., the reflection region 102a and the transparent electrode 108 (see FIG. 9) when the substrates 104, 105 are pressed to combine them. The carving refuse is conductive and causes leak current to run between the substrates. The leak current increases remarkably if the smaller cell gap is less than 3 μm.
As in the above, the cell thickness is more difficult to control and defects, such as current leak, are more likely to develop in the liquid crystal display with two different cell gaps, small and large, than in the liquid crystal display with a single cell gap.
The distributed spherical spacers 103 disturb the alignment of the liquid crystal in proximity thereto and cause light leak therearound, which reduces pixel contrast of the liquid crystal display.
In the transmission/reflection hybrid liquid crystal display, if the electrode in the reflection region 102a is configured in MRS (Micro Reflector Structure) with undulations that double as a reflector plate as shown in FIG. 11, the cell gap varies depending on where the spherical spacer 103 is located in the undulations in the reflection region 102a. 
A technology recently getting peoples' attention is to distribute columnar spacers made of resin or other material instead of spherical spacers.
The following briefly introduces some conventional examples of columnar spacers employed in the reflective liquid crystal display or the transmissive liquid crystal display.
Japanese Laid-Open Patent Application No. 50-39095/1975 (Tokukaisho 50-39095; published on Apr. 10, 1975), Japanese Laid-Open Patent Application No. 59-143124/1984 (Tokukaisho 59-143124; published on Aug. 16, 1984), Japanese Laid-Open Patent Application No. 56-33626/1981 (Tokukaisho 56-33626; published on Apr. 4, 1981), and Japanese Laid-Open Patent Application No. 56-99384/1981 (Tokukaisho 56-99384; published on Aug. 10, 1981), among others, disclose spherical spacers being distributed at given places on one of the substrates composing the liquid crystal cell by means of photolithography and other techniques.
According to the disclosure of these patent applications, ITO electrodes, columnar spacers, and an alignment film are formed on a glass substrate to which color filters are provided. These members may be provided in any order, as long as it is the alignment film that is formed last of all.
Japanese Laid-Open Patent Application No. 61-173221/1986 (Tokukaisho 61-173221; published on Aug. 4, 1986) discloses columnar spacers made of an organic resin such as polyimide. Japanese Laid-Open Patent Application No. 54-4154/1979 (Tokukaisho 54-4154; published on Jan. 12, 1979) discloses columnar spacers made of an inorganic resin, such as SiO2, or a metal.
Aforementioned Patent Application No. 56-99384/1981 discloses photosensitive columnar spacers. Japanese Laid-Open Patent Application No. 63-116126/1988 (Tokukaisho 63-116126; published on May 20, 1988) discloses columnar spacers made of a black resin.
Further alternatives are suggested to form columnar spacers by depositing color filters. It is known that, for example, the columnar spacers are disposed between electrodes or a non-transmissive member such as an electrode or on a black matrix of a color filter (see Japanese Laid-Open Patent Application No. 62-239126/1987 (Tokukaisho 62-239126; published on Oct. 20, 1987) for an example). The shape may vary and include dot-shaped (adductor-shaped) ones and stripes as disclosed in aforementioned Japanese Laid-Open Patent Application No. 63-116126/1988.
The columnar spacers are advantageous in that they can be fabricated by photolithography, printing, transfer, etc. and their density and the size of the individual spacer can therefore be set to any given values.
Japanese Laid-Open Patent Application No. 61-267736/1986 (Tokukaisho 61-267736; published on Nov. 27, 1986) discloses the inclusion of 0.1 to 100 protrusions, 20 μm long or less on each side, for every 1 mm2 of the area of the substrate to provide an extra shock resistance to ferroelectric liquid crystal.
U.S. Pat. No. 5,978,061 (corresponding to Japanese Laid-Open Patent Application No. 9-73093/1997 (Tokukaihei 9-73093; published on Mar. 18, 1997)), Japanese Laid-Open Patent Application No. 9-73099/1997 (Tokukaihei 9-73099; published on Mar. 18, 1997), and Japanese Laid-Open Patent Application No. 9-73088/1997 (Tokukaihei 9-73088; published on Mar. 18, 1997) specifies the ratio of the area occupied by columnar spacers per unit area of the substrate for the stable supply in the cell gap of the liquid crystal cell and the prevention of bubble formation at low temperatures.
However, none of the patent applications listed above sufficiently discusses the distribution structure, of columnar spacers, which provides two or more different, stable cell gaps in a liquid crystal display.
Further, none of the patent applications discusses an arrangement to prevent the alignment from developing a defect in proximity to columnar spacers when the alignment film is subjected to a rubbing treatment after the columnar spacers are formed.
The present invention addresses these conventional problems and has an objective to provide a liquid crystal display that has two or more different, stable cell gaps and still exhibits high display quality.